1. Field of the Invention
This invention pertains generally to ion exchange membranes, and more particularly to functionalization of porous materials to produce proton exchange membranes that maintain conductivity at temperatures above 100° C.
2. Description of Related Art
Research on proton exchange membranes (PEMs) has focused on improving proton conductivity at elevated temperatures (e.g., >100° C.). Current state-of-the-art proton exchange membranes are the product of over 30 years of steady improvement to performance and lifetime of perfluorosulfonic acid (PFSA) or other organic polymer membranes. These polymer membranes generally contain hydrophilic regions of interconnected acidic (e.g., sulfonic acid) clusters surrounded by a hydrophobic matrix (e.g., a fluorinated polymer, such as perfluorinated ethylene). Under atmospheric pressure and near 100% relative humidity, for example, PFSA membranes have been reported to possess proton conductivities up to 0.1 S/cm at 80° C. However, unless a pressurized feed stream (˜8 atm) containing saturated water vapor is used, the membrane will dehydrate and lose sufficient proton conductivity at temperatures above 100° C. Therefore, PFSA membrane implementation into PEM fuel cells requires relatively low-temperature (<80° C.) operation.
Substantial efforts have been made to develop new ion conducting membranes, especially proton exchange membranes, for use at temperatures above 100° C. Most research in this area focuses on the development of (1) sulfonated polymer membranes, (2) acid-base polymer membranes, (3) modified PFSA and sulfonated polymer membranes containing inorganic particles, or (4) hybrid organic/inorganic membranes. Sulfonated hydrocarbon polymers such as polysulfones, polyether-etherketone (PEEK), polyimides, polyphenylsulfide, and polybenzimidazoles (PBI) have generally been suggested as low cost alternatives to PFSA membranes under low-temperature operation. A few polymers, including sulfonated PBI and poly(arylene ether sulfone) (BPSH), have shown relatively high proton conductivity and thermal stability at elevated temperatures. However, most of these sulfonated hydrocarbon polymers exhibit poor chemical stabilities compared to PFSA polymers. Acid-base polymer membranes have also been investigated. Polymers bearing basic sites such as ether, alcohol, imine, amide, or imide groups act as a solvent in which strong acids readily dissociate and allow for high proton conductivity. H3PO4-doped PBI polymers have obtained proton conductivity values of ˜10−2 S/cm at temperatures between 100-200° C. and show good thermal and chemical stability.
In addition to the development of new entirely polymeric materials, inorganic-organic composite membranes, such as those containing hydrophilic zeolite, silica, or titania particles introduced into proton conductive polymers tend to retain water at higher temperatures and display modestly improved proton conductivities. PFSA composites containing SiO2, zirconium oxide, titanium oxide, and zirconium phosphate particles have all shown enhanced water retention and allow for reduced humidification at operating temperatures in the range of 100-120° C. Likewise, improved water retention and proton conductivity have been obtained through the incorporation of inorganic particles into other proton exchange membrane materials. Alternatively, porous inorganic particles have been functionalized by proton conducting species or filled with proton conducting polymers. While these approaches have generally increased bulk hydrophilicity of the respective materials, they have not led to appreciable improvements in bulk proton conductivity at temperatures above 100° C., largely because the continuous polymeric host matrices present the same hydration-dependent conduction barriers as in the non-composite materials.
Another method of incorporating inorganic materials into a polymer matrix involves directly copolymerizing metal alkoxides, such as tetraethoxysilane or organic substituted alkoxides, with polymers to produce covalent bonding at the organic/inorganic interface. These materials offer the benefit of low temperature sol-gel processing, good thermal and chemical stability, and proton conductivity properties of 10−3 S/cm to 10−2 S/cm at operating temperatures up to 160° C., but only under saturated humidity conditions and with disordered structures, including the absence of well-defined pores or channels.